sql - MySQL very slow for alter table query

Question

Why is it taking more than an hour to simply update this table to add a column? This table has 15M rows. It has 2 indexes and a single key primary key. The ALTER TABLE query has been in "copy to tmp table" state for 1 hour 15 minutes now.

ALTER TABLE `frugg`.`item_catalog_map` 
ADD COLUMN `conversion_url` TEXT NULL DEFAULT NULL

Table:

mysql> describe item_catalog_map;
+------------------------+---------------+------+-----+---------+-------+
| Field                  | Type          | Null | Key | Default | Extra |
+------------------------+---------------+------+-----+---------+-------+
| catalog_unique_item_id | varchar(255)  | NO   | PRI | NULL    |       |
| catalog_id             | int(11)       | YES  | MUL | NULL    |       |
| item_id                | int(11)       | YES  | MUL | NULL    |       |
| price                  | decimal(10,2) | YES  |     | 0.00    |       |
+------------------------+---------------+------+-----+---------+-------+

mysql> show index from item_catalog_map;
+------------------+------------+----------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+
| Table            | Non_unique | Key_name             | Seq_in_index | Column_name            | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+------------------+------------+----------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+
| item_catalog_map |          0 | PRIMARY              |            1 | catalog_unique_item_id | A         |    15485115 |     NULL | NULL   |      | BTREE      |         |
| item_catalog_map |          1 | IDX_ACD6184FCC3C66FC |            1 | catalog_id             | A         |          18 |     NULL | NULL   | YES  | BTREE      |         |
| item_catalog_map |          1 | IDX_ACD6184F126F525E |            1 | item_id                | A         |    15485115 |     NULL | NULL   | YES  | BTREE      |         |
+------------------+------------+----------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+

Answer 1

MySQL’s ALTER TABLE performance can become a problem with very large tables. MySQL performs most alterations by making an empty table with the desired new structure, inserting all the data from the old table into the new one, and deleting the old table. This can take a very long time, especially if you’re short on memory and the table is large and has lots of indexes. Many people have experience with ALTER TABLE operations that have taken hours or days to complete.

Anyway if you need to proceed with alter table, maybe the following resources could help you:

• https://www.percona.com/doc/percona-toolkit/2.2/pt-online-schema-change.html
• https://github.com/soundcloud/lhm
• https://githubengineering.com/gh-ost-github-s-online-migration-tool-for-mysql/